In large, complex industrial control systems, it has been the practice to provide redundancy in various components of the system so that failure of one component of the system will not result in failure of the system. For example, redundancy has been provided in transmission lines, and in data processors. The present invention is directed to an improved system for providing redundancy for what is known as an analog input/output board wherein the analog I/O board receives from a data processor digital signals representing analog values to control an analog signal responsive unit such as a valve or a motor speed setting. In addition, the analog I/O board receives analog input signals and converts the analog input signals to digital values which are transmitted back to the data processor.
In prior art systems, redundancy has been provided for analog-to-digital converters, but the systems of the prior art were simplistic in nature and the switching to a redundant analog-to-digital converter was carried out under the control of the data processor or solely in response to a condition wherein the analog-to-digital converter currently operating failed to provide an operating signal accurately tracking the digital input.